Figure 1

(a) Crystal structure of the layered ${\mathrm{LiCoO}}_2$ ($\alpha$-${\mathrm{NaFeO}}_2$ structure, $R\overline{3}m$). Bottom: $ab$ plane. A triangle consisting of surface Li atoms is shown. (b) Constant-current STM image of a ${\mathrm{Li}}_{0.66}{\mathrm{CoO}}_2$ single crystal measured at 5 K, showing hexagonal lattice of Li ions at the surface; $4.8\times 4.8{\mathrm{nm}}^2$, sample bias voltage $V_s=-1.0\mathrm{V}$, and tunneling current $I_t=0.2\mathrm{nA}$. The atomic spacing ($\sim 0.3\mathrm{nm}$) is consistent with the Li-Li distance shown as a triangle. (c) Averaged differential conductance $dI/dV$ spectrum taken at surface Li ions in (b). Set point: $I_t=0.2\mathrm{nA}$, $V_s=-1.0\mathrm{V}$. (d) STM image at $V_s=+0.8\mathrm{V}$ measured at the same location as in (b). (e) STM image at the same location as in (b), subsequently taken after (d) at $V_s=-1.0\mathrm{V}$.Reuse & Permissions

Figure 2

[(a) and (b)] Bias-dependent STM image of ${\mathrm{Li}}_{0.66}{\mathrm{CoO}}_2$ at $V_s=-0.6$ and $+0.6\mathrm{V}$, showing ($3\times 3$) structures, respectively; $6\times 6{\mathrm{nm}}^2$, $I_t=0.3\mathrm{nA}$. The autocorrelation image of (b) is shown in the inset. (c) Comparison of line profile of the STM image at $-0.6$ and $+0.6\mathrm{V}$ in (a) and (b). For clarity, the line profile at $+0.6\mathrm{V}$ is shifted vertically. (d) Averaged $dI/dV$ spectrum taken in (a). Set point: $I_t=0.3\mathrm{nA}$, $V_s=-0.8\mathrm{V}$.Reuse & Permissions

Figure 3

(a) Schematic structure of the ($3\times 3$) ordering at ${\mathrm{Li}}_{0.66}{\mathrm{CoO}}_2$ surface. Left: surface structure, right: subsurface structure. The surface is terminated with ${\mathrm{CoO}}_2$ layer, in which ${\mathrm{Co}}^{3+}/{\mathrm{Co}}^{4+}=1:2$ is realized due to the charge balance effect. The surface oxygen atom O1 lies among three adjacent ${\mathrm{Co}}^{3+}$ ions, O2 is surrounded by one ${\mathrm{Co}}^{3+}$ and two ${\mathrm{Co}}^{4+}$ ions, and O3 lies among adjacent three ${\mathrm{Co}}^{4+}$ ions. $\mathrm{O}{3}^{\prime }$ is also surrounded by three ${\mathrm{Co}}^{4+}$ ions but has an outer atomic arrangement different from that of O3. Oxygen atoms located below Co ions are illustrated as smaller circles. The ($3\times 3$) unit cell is shown as a thick solid line. In the subsurface layer (right), Li vacancies and ${\mathrm{Co}}^{4+}$ ions are introduced to satisfy $x=0.66$ (i.e., ${\mathrm{Co}}^{3+}/{\mathrm{Co}}^{4+}=2:1$) and placed to form the $(\sqrt{3}\times \sqrt{3})R30°$ structure. Li ions and vacancies located below this layer are illustrated as smaller circles. [(b) and (c)] Simulated integrated DOS images from spin-polarized DFT calculations at $V_s=-0.8$ and $+0.8\mathrm{V}$, respectively. The position of each surface oxygen atom is indicated using the same color as in (a). (d) Simulated partial DOSs of surface oxygen atoms O1 and O3 in (a). Majority and minority spin components are added together. (e) Simulated partial DOSs of ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ ions in (a). Majority and minority spin components are added together.Reuse & Permissions

Figure 4

[(a) and (b)] Bias-dependent STM images of ${\mathrm{Li}}_{0.66}{\mathrm{CoO}}_2$ at $V_s=-1.0$ and $+0.8\mathrm{V}$, showing disordered structures, respectively. Both images were taken at the same position; $7\times 7{\mathrm{nm}}^2$, $I_t=0.3\mathrm{nA}$. There exist three different protrusions, which appear only at $-1.0\mathrm{V}$ (triangles) and $+0.8\mathrm{V}$ (squares) and at both voltages (circles). To compare between these two images, the identical positions are indicated as crosses. (c) Autocorrelation image of (a). (d) Averaged $dI/dV$ spectrum taken in (a). Set point: $I_t=0.2\mathrm{nA}$, $V_s=-1.0\mathrm{V}$.Reuse & Permissions